Add support for the AWS_MSK_IAM SASL mechanism

[Dawnflash]

AWS_MSK_IAM is an OAUTHBEARER-based SASL mechanism for accessing AWS MSK clusters using AWS IAM.
The OAUTH token is supplied by AWS APIs so it fits nicely into the already written OAUTH token provider.

Uses https://github.com/aws/aws-msk-iam-sasl-signer-go with the region inferred by the AWS SDK. I decided not to add explicit region configuration to the config to keep it cleaner but that would be an easy addition as well.

Resolves #198.

[birdayz]

nice and clean. thank you!

[yermulnik]

@birdayz Is this going to be wrapped into a new kaf release please?

[yermulnik]

@Dawnflash I've just wanted to try and use kaf to connect to MSK using IAM Auth, though I couldn't figure out how to use AWS CLI profile for authentication. Could you please assist? Thanks in advance.

[Dawnflash]

@yermulnik try this

SASL:
  username: ""
  mechanism: AWS_MSK_IAM
TLS: null
security-protocol: SASL_SSL

To use a profile just export the AWS_PROFILE envvar.

[yermulnik]

@Dawnflash That worked! Thanks a lot.